Various gases such as natural gas, coke oven gas or cracked gas generated during steps of refining petroleum, etc. have been widely utilized industrially as fuels or synthetic raw materials. However, gases containing gaseous impurities, in particular, acid gases, cannot be utilized as fuels or synthetic raw materials as they are.
For example, a representative composition of natural gas is:
______________________________________ hydrocarbon 75-96 (vol %, hereafter the same) H.sub.2 S 0-15 CO.sub.2 4-10 N.sub.2 0-0.5 ______________________________________
In order to liquify natural gas, however, it is necessary that CO.sub.2 and H.sub.2 S be reduced to 100 ppm or less and 50 ppm or less, respectively. Further, in order to utilize natural gas as a fuel, it is indispensable to reduce H.sub.2 S to 5 ppm or less. Further, in ammonia synthesis industry, various cracked gases are utilized as a source for H.sub.2 ; for example, a representative composition of natural gas is:
______________________________________ H.sub.2 S 80 (vol %, hereafter the same) CO.sub.2 less than 1 N.sub. less than 1 CO.sub.2 19 CH.sub.4 less than 1 ______________________________________
In case that the cracked gas is utilized as a raw material, it is necessary that the content of CO.sub.2 be reduced to 0.3% or less.
As the prior art, there are, for example, U.S. Pat. Nos. 2,649,166, 3,362,133, 3,737,392, 4,044,100 and 4,581,154, and Japanese Patent Laid-Open Application No. 156629/85. These publications teach that certain compositions of polyethylene glycol dimethyl ether are effective as chemicals for absorbing acid gases.
In particular, an absorbent of this type commercially available under the trademark of Selexol Solvent released from Norton Company in America has been utilized for operation of treating gases in various areas.
Acid gas absorbents are reproduced in a reproduction step upon contact with air or N.sub.2 gas at temperatures as high as 70.degree. C. or more and recycled to use them.
It is important that the chemicals be reproduced in the reproduction step at temperatures as high as possible thereby to minimize retention of acid gases contained in the absorbents.
This is because, as the residual gas is minimized as low as possible, an absorbing ability in an absorbing step is improved and purity of purified gas becomes high so that gas throughput capacity of the overall process can be substantially improved.
In order to achieve this object, it is indispensable to develop absorbents having excellent stability at high temperatures and such has been strongly desired in related fields.
However, acid gas absorbents disclosed heretofore cause decomposition in contact with CO.sub.2 or H.sub.2 S at a temperature of, for example, 100.degree. C. and are decomposed to compounds having a poor acid gas-absorbing capability, for example, methanol, polyethylene glycol monomethyl ether, etc.